1. Field of the Invention
The invention relates to ion source elements and, particularly, to a stable ion source element.
2. Discussion of Related Art
Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). CNTs are electrically conductive along their length, are chemically stable, and can each have a very small diameter (much less than 100 nanometers) and a large aspect ratio (length/diameter). Due to these and other properties, it has been suggested that carbon nanotubes can play an important role in a variety of fields, such as microscopic electronics, field emission devices (FED), scanning electron microscopes (SEM), transmission electron microscopes (TEM), etc.
One conventional type of ion source element includes a cold cathode with a CNT film formed thereon, a grid electrode arranged above the cold cathode, and an ion accelerator arranged above the grid electrode (i.e., the grid electrode is positioned between the cold cathode and the ion accelerator). The CNT film acts as an electron emitter for the ion source element, and, consequently, the ion source element has a low power consumption and a low evaporation rate. In operation, electrons emit from the CNT film and travel to the grid electrode, and such electrons are eventually collected by the grid electrode. The ion source element operated in a certain vacuum level, and there are still some gas molecules and/or atoms therein. In their travel, electrons bombard with the gas molecules and/or atoms and, thereby, create gas ions. The gas ions and electrons bombard with the CNT film or/and interact with the CNT film, and then the CNT film can be locally destroyed and/or transformed. Therefore, the ion source element can be unstable, over an extended period of use.
What is needed, therefore, is an ion source element that is stable and suitable for a variety of applications.